The 55 Cancri system reassessed

Abstract

Orbiting a bright, nearby star the 55 Cnc system offers a rare opportunity to study a multiplanet system that has a wide range of planetary masses and orbital distances. Using two decades of photometry and spectroscopy data, we have measured the rotation of the host star and its solar-like magnetic cycle. Accounting for this cycle in our velocimetric analysis of the system allows us to revise the properties of the outermost giant planet and its four planetary companions. The innermost planet 55 Cnc e is an unusually close-in super-Earth, whose transits have allowed for detailed follow-up studies. Recent observations favor the presence of a substantial atmosphere yet its composition, and the nature of the planet, remain unknown. We combined our derived planet mass (M p = 8.0±0.3 M Earth ) with refined measurement of its optical radius derived from HST/STIS observations (R p = 1.88±0.03 R Earth over 530-750 nm) to revise the density of 55 Cnc e (ρ = 6.7±0.4 g cm −3 ). Based on these revised properties we have characterized possible interiors of 55 Cnc e using a generalized Bayesian model. We confirm that the planet is likely surrounded by a heavyweight atmosphere, contributing a few percents of the planet radius. While we cannot exclude the presence of a water layer underneath the atmosphere, this scenario is unlikely given the observations of the planet across the entire spectrum and its strong irradiation. Follow-up observations of the system in photometry and in spectroscopy over different time-scales are needed to further investigate the nature and origin of this iconic super-Earth.

Atmospheric compositions and observability of nitrogen-dominated ultra-short-period super-Earths

“…Despite observational and modeling efforts (Demory et al 2016b;Angelo & Hu 2017;Hammond & Pierrehumbert 2017;Bourrier et al 2018;Miguel 2019), 55 Cnc e remains one of the most mysterious worlds discovered to date. Its large day-night temperature gradient and an eastwards shifted hot spot favours a scenario where it is surrounded by a high-mean-molecular weight atmosphere, possibly composed mainly of N 2 .…”

“…Another concern is whether USP super-Earths would be able to retain sufficient hydrogen to form detectable levels of volatile species such as HCN, H 2 O, CH 4 or unsaturated hydrocarbons. Due to erosion, it is assumed that USP planets would not be able to sustain a large hydrogen or helium envelope over a long period of time (Bourrier et al 2018). Failure to detect hydrogen in the exosphere of 55 Cnc e through Ly-α (Ehrenreich et al 2012) further reinforces this point.…”

“…For our photochemical calculations we have constructed a synthetic spectrum for 55 Cancri, a 0.98 solar radii star with T eff ∼5200 K (Bourrier et al 2018;Yee et al 2017;von Braun et al 2011). It is composed of the synthetic ATLAS spectrum for a star with T e f f = 5250 K, with coadded UV spectra extensions from the International Ultraviolet Explorer observations of a similar star -HD 10780 (Kurucz 1979;Rugheimer et al 2013).…”

“…For this comparison we use the available measured spectra of 55 Cancri from Crossfield (2012), which has higher flux at 5 µm compared to our generated PHOENIX model. The variability of 55 Cnc e is a known issue, and several possible explanations have been published in the last years, ranging from large scale volcanic plumes, a gas-dust torus, reflective grains or magnetic interaction, to other effects (Demory et al 2016a,b;Bourrier et al 2018;Tamburo et al 2018;Sulis et al 2019;Folsom et al 2020). More observations are needed to decipher the issue.…”

“…While the analysis of spectroscopic HST/WFC3 observations by Tsiaras et al (2016a) suggest that the atmosphere might be light-weighted, retaining a significant amount of hydrogen and helium, the interior structure modelling by Bourrier et al (2018) excludes any possibility of an extended H/He gas envelope. This assertion is supported by the nondetection of H or H 2 O in the atmosphere by Ehrenreich et al (2012); Esteves et al (2017).…”